1. Field of the Invention
The object of the present invention is an improved golf club shaft and it is related more specifically to a shaft of a composite material whose flexibility distribution has been optimized in order to better the performance of the club. The invention is also related to a golf club equipped with such a shaft.
2. Discussion of Background and Material Information
Prior art club shafts are generally shaped like a truncated tubular section whose biggest diameter is found at the upper end (generally called the "butt"), adapted to receive the grip of the club and the smallest diameter is located at the lower end (called the "tip"), adapted to receive the club head. Flexional rigidity, which is the product of Young's modulus for the material (E) and the inertia of the section (1) decreasing substantially exponentially from the "butt" end towards the "tip" end, whereby 1=.pi. (D.sup.4 -d.sup.4)/64 (and whereby D=external diameter and d=internal diameter of the shaft) (FIG. 1). The rigidity values (EI) at the "butt" end of the shaft vary depending on the commercial availability of a shaft, from 60 to 100 N.multidot.m.sup.2 (Newton.multidot.meters.sup.2), but the rate of the curve remains substantially the same.
For conventional shafts selected from among those whose "butt" portion has low rigidity values, a substantial increase of the head speed is obtained, and consequently, the launching speed of the ball. However, flexibility is distributed along the entire length of the shaft due to conventional shaft design as is shown by the rate of the curve (C2) of FIG. 1. Shaft deformation speed during swing movement (and more specifically during the "release" phase before impact) adds to the drive speed of the shaft generated by the player and produces an increase of the order of 1 to 2 m/s (meters per second).
However, the speed of rotation ("back spin") and the angle at which the ball leaves with such so-called "flexible" shafts are greater than with other shafts. The flight paths of the balls are therefore higher and the ball stops more quickly upon striking the ground; this means that in terms of distance, the results are no greater than those achieved with conventional shafts having standard rigidity (curve C1). Rectification by decreasing the surface angle (called "static" loft) is not enough to adequately modify the values of these two parameters in order to improve club performance.
U.S. Pat. No. 4,319,750 discloses a shaft made of a composite material whose first 250 mm (millimeters) from the "butt" end are rendered more flexible with respect to the remainder of the shaft. This zone of reduced flexional rigidity corresponds to the gripping area of the club. The hands of the player surround or embed the club in this area, thereby resisting deformation in this area. Consequently, a greater flexibility in this area is hardly efficient, and the player is all the more disturbed because he has a tendency to fasten tightly onto the grip of the club in order to resist the deformation thus produced.